1. Field of the Invention
This invention relates to an improvement in a plasma cleaning method for removing a thin film deposited on the inner face of a vacuum vessel and the surfaces of members in the vacuum vessel in a vacuum processing system such as a plasma enhanced chemical vapor deposition (PECVD) system used in a semiconductor device manufacturing process, for example.
2. Description of the Related Art
FIG. 5 is an illustration showing a schematic configuration of a plasma enhanced chemical vapor deposition system as an example of a conventional vacuum processing system.
The plasma enhanced chemical vapor deposition system shown in FIG. 5 consists mainly of a vacuum vessel 1 provided with an exhaust channel 11, a gas introduction mechanism 2 for introducing a predetermined gas into the vacuum vessel 1, a power supply mechanism 3 for energizing the introduced gas for forming plasma, and a substrate stage 4 for placing a substrate 40 on which a thin film is to be deposited.
The system in FIG. 5 carries the substrate 40 into the vacuum vessel 1 through a gate valve (not shown) and places the substrate 40 on the substrate stage 4. After evacuating the air in the vacuum vessel 1 through the exhaust channel 11, the system introduces a predetermined gas by the gas introduction mechanism 2. Next, the system applies energy of high-frequency electromagnetic wave power to the gas in the vacuum vessel 1 by the power supply mechanism 3 for forming plasma. Then, the system deposits a predetermined thin film on the surface of the substrate 40 by a vapor reaction enhanced by the plasma. For example, if mono-silane gas and oxygen gas are introduced by the gas introduction mechanism 2, the plasma causes a decomposition reaction, and a silicon oxide thin film is deposited on the surface of the substrate 40.
When the thin film deposition is repeated in the plasma enhanced chemical vapor deposition system, a phenomenon occurs in which a silicon oxide thin film is also deposited on the surface of the substrate stage 4 exposed to the plasma and the inner face of the vacuum vessel 1. As the thin film is deposited on them, it will soon peel off due to the internal stress of the thin film, causing fine particulates to occur. If the fine particulates adhere to the silicon oxide thin film on the substrate 40, it causes a surface contamination to occur, degrading the commodity value of the silicon oxide thin film.
Such a problem occurs not only in the plasma enhanced chemical vapor deposition system, but also in a vacuum processing system such as a plasma etching system. That is, an etched material is deposited on the surface of a substrate stage and the inner wall of a vacuum vessel, depositing a thin film, which will peel off, causing fine particulates to occur, damaging circuitry on the substrate.
To suppress such a deposited film peeling, a plasma cleaning technique is generally used for etching and removing the unwanted deposited film before it peels off and contaminates a deposited thin film. In the technique, mixed gas of perfluorocarbon 14 gas (CF4):oxygen gas=80:20 or so is introduced into the vacuum vessel 1 by the gas introduction mechanism 2, plasma of the perfluorocarbon 14 gas and oxygen gas is generated, and the unwanted deposited film is etched and removed by the action of the plasma. That is, radial CFx (x=1, 2, or 3), CFx ions (x=1, 2, or 3), radial fluorine, or fluorine ions are generated in the plasma, and the fluorine radicals or ions react with the unwanted deposited film to yield volatile material, which is then discharged through the exhaust channel 11, thereby removing the thin film.
In the vacuum processing system in which the plasma cleaning is executed, the members are situated according to the concept that plasma is formed essentially for substrate processing. Thus, if the substrate stage is positioned such that it is exposed to too much plasma, damage to the substrate will result. Therefore, the substrate stage is located at a position considerably distant from the plasma forming place. In this case, when the plasma cleaning is executed, the plasma density in the vicinity of the surface of the substrate stage is unavoidably lowered. As a result, removal of the deposited film on the surface of the substrate stage requires a prolonged duration for plasma cleaning.
Particularly, in a thin film deposition system such as a plasma enhanced chemical vapor deposition system, a thin film is not deposited on the area covered with the substrate during the processing, which will be hereinafter referred to as the "placement area", on the surface of the substrate stage, but is deposited on an uncovered surface area, which will be hereinafter referred to as the "non-placement area". In recent years, various attempts have been made to enhance the film deposition rate. Such attempts result in not only high-rate film deposition on the substrate, but also deposition of a thick film on the non-placement area of the substrate stage.
Since only low-density plasma is exposed to the non-placement area of the substrate stage on which such a thick film is deposited, a serious problem of prolonging the plasma cleaning and lowering the availability of the system arises.
In plasma cleaning of the chamber, such as the inner face of the vacuum vessel, other than the substrate stage, the etch rate cannot sufficiently be increased and the plasma cleaning time period cannot be shortened.